The human body imposes stringent requirements on electrical conductors that are implanted within it. Leads inserted in a living body that form a part of a stimulation and/or measuring/sensing or drug delivery device are subjected to almost continuous flexure during body movement. Also, a living body constitutes a site that is chemically and biologically hostile to anything that invades it. Consequently, leads ideally have good deformability, particularly bending ability, high fatigue resistance, corrosion resistance, and a high degree of electric conductance.
U.S. Pat. No. 4,640,983 (Comte) discloses a lead body having at least one spiral which is formed from at least one conductor. Each conductor consists of wires that are stranded together to form a bundle. The wires belonging to a conductor can all consist of the same material or of different materials, one of which has a large breaking strength and the other one having high electric conductivity. One embodiment discloses inner and outer coils wound in the same direction and separated by an insulating sheath.
U.S. Pat. No. 5,483,022 (Mar) discloses a lead body having at least one helical coil formed from at least one electrical cable. Each cable is formed from several wires twisted in a ropelike configuration. Each wire is a composite of a core of a highly conductive material and a coveting of a strong and biocompatible material, with all adjacent wires belonging to the same cable in un-insulated contact with each other. One embodiment discloses inner and outer coils wound in the same direction and separated by an insulating sheath.
U.S. Pat. No. 6,978,185 (Osypka) discloses a lead body having electrically active and inactive elements helically wound together. The active elements are separated from each other by the inactive elements. One embodiment discloses inner and outer coils wound in opposite directions and separated by an insulating sheath.
When winding coiled lead bodies the conductors or cables are typically strained beyond the yield point so the conductors or cables retain a coiled configuration. When the winding force is removed, the conductors or cable provide a self-supporting coil.
This construction method, however, limits the number of individual conductors in a given lead body. First, current multi-conductor coils permit essentially zero elongation. Second, the flex fatigue life of the conductors is reduced if too many conductors are located in different planes for a given coil diameter. As a result, lead bodies are typically limited to an inner coil with about four conductors and an outer coil with four to eight conductors.